The present invention relates generally to ultrasonic transducers, and, more particularly, to aperture control and apodization in a micro-machined ultrasonic transducer.
Ultrasonic transducers have been available for quite some time and are useful for interrogating solids, liquids and gasses. One particular use for ultrasonic transducers has been in the area of medical imaging. Ultrasonic transducers are typically formed of piezoelectric elements. The elements typically are made of material such as lead zirconate titanate (abbreviated as PZT), with a plurality of elements being arranged to form a transducer assembly. The transducer assembly is then further assembled into a housing possibly including control electronics, in the form of electronic circuit boards, the combination of which forms an ultrasonic probe. This ultrasonic probe, which may include acoustic matching layers between the surface of the PZT transducer element or elements and the probe body, may then be used to send and receive ultrasonic signals through body tissue.
One limitation of PZT devices is that the acoustic impedance is approximately 30-35 MRayls (kg/m2s), while the acoustic impedance of the human body is approximately 1.5 MRayls. Because of this large impedance mismatch acoustic matching layers are needed to match the PZT impedance to the body impedance. Acoustic matching layers work using a xc2xc wave resonance principle and are therefore narrow band devices, their presence thus reducing the available bandwidth of the PZT transducer. In order to achieve maximum resolution, it is desirable to operate at the highest possible frequency and the highest possible bandwidth.
In order to address the shortcomings of transducers made from piezo-electric materials, a micro-machined ultrasonic transducer (MUT), which is described in U.S. Pat. No. 5,619,476 to Haller, et al., has been developed. Micro-machined ultrasonic transducers address the shortcomings of PZT transducers by, among other attributes, being fabricated using semiconductor fabrication techniques on a silicon substrate. The MUT""s are formed using known semiconductor manufacturing techniques resulting in a capacitive non-linear ultrasonic transducer that comprises, in essence, a flexible membrane supported around its edges over a silicon substrate. By applying contact material to the membrane, or a portion of the membrane, and to the silicon substrate and then by applying appropriate voltage signals to the contacts, the MUT may be energized such that an appropriate ultrasonic wave is produced. Similarly, with the application of a bias voltage, the membrane of the MUT may be used to generate receive ultrasonic signals by capturing reflected ultrasonic energy and transforming that energy into movement of the membrane, which then generates a receive signal. When imaging the human body, the membrane of the MUT moves freely with the imaging medium, thus eliminating the need for acoustic matching layers. Therefore, transducer bandwidth is greatly improved.
Another limitation of PZT transducers is that the apparatus required for switching the transducers within an array are bulky and consume valuable space. For example, in order to switch a PZT transducer array, a number of switching elements are typically provided on an electronic circuit assembly, the assembly integrated with the transducer array into a housing to form an ultrasonic probe. These probes tend to be large and bulky due, in large part, to the presence of the switching apparatus.
Therefore it would be desirable to have an ultrasonic transducer array in which little space is occupied by the switching apparatus.
The invention provides aperture control and apodization in a micro-machined ultrasonic transducer (MUT) by using switching apparatus located on the same substrate as the MUT.
In architecture, the present invention may be conceptualized as MUT array, comprising at least one MUT element, the MUT element including at least one MUT cell, the MUT cell formed on a substrate, and control means, located on the substrate and associated with the MUT element, for controlling the MUT array.
In another aspect, the present invention may be conceptualized as a MUT array, comprising at least one MUT element, the MUT element including at least one MUT cell, the MUT cell formed on a substrate, and control circuitry associated with the MUT element, the control circuitry configured to supply a bias voltage to the MUT element.
In yet another aspect, the invention may be conceptualized as a MUT array, comprising at least one MUT element, the MUT element including a plurality of MUT cells, the MUT cells having a variable gain, such that the gain of MUT cells located toward a center of the MUT element is greater than the gain of the MUT cells located toward a periphery of the MUT element.
The present invention may also be conceptualized as providing a method for controlling a MUT array, the MUT array including at least one MUT element formed on a substrate, the MUT element including at least one MUT cell, the method comprising the steps of: forming control means on the substrate, the control means associated with the MUT element, the control means formed on the substrate, and selectively energizing the MUT element in the array by activating the control means.
In another aspect, the invention may be conceptualized as providing a method for controlling a MUT array, the MUT array including at least one MUT element, each MUT element including a plurality of MUT cells, the method comprising the steps of: forming the MUT element on a substrate, and supplying a bias voltage to the MUT element in order to control an acoustical profile of the MUT element.
In yet another aspect, the invention may be conceptualized as providing a method for controlling a MUT array, the array including at least one MUT element, the MUT element including a plurality of MUT cells, the method comprising the steps of: varying a gain of each MUT cell such the gain of MUT cells located toward a center of the MUT element is greater than the gain of the MUT cells located toward a periphery of the MUT element.